Humidity indicators

ABSTRACT

Silica-based carrier impregnated with iron(III) or iron(II) salts functions as a humidity indicator giving a yellow/amber to nearly colourless colour change as the carrier humidifies.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Ser. No. 09/959,121, filedOct. 22, 2001, now U.S. Pat. No. 6,753,184, issued Jun. 22, 2004, whichis the National Phase of PCT/GB00/01390, filed Apr. 12, 2000 designatingthe United States, and which was published in English. Theseapplications, in their entirety, are incorporated herein by reference.

This invention relates to silica-based humidity indicators.

Cobalt chloride indicator gels are used in a range of applications, e.g.to indicate moisture change in gas drying columns. Other dryingapplications include their use in transformer breathers, tank breathers,in the protection of electronics and telecommunication systems and inlaboratory desiccators. It is estimated that approximately 2000 tonnesof cobalt chloride indicator gel are used annually on a global basis.

Cobalt-containing gels for use as humidity indicators have beendisclosed in U.S. Pat. No. 2,460,071 (disclosing cobalt chloride), U.S.Pat. No. 2,460,069 (disclosing cobalt bromide), U.S. Pat. No. 2,460,073(disclosing cobalt iodide), U.S. Pat. No. 2,460,074 (disclosing cobaltthiocyanate), U.S. Pat. No. 2,460,065 (disclosing cobalt sulphate) andU.S. Pat. No. 2,460,070 (disclosing cobalt phosphate).

Indicator silica gel is currently produced by impregnating humidifiedsilica gel or a silica hydrogel with a cobalt chloride solution toproduce a dry granular end-product which contains a minimum of 0.5%cobalt chloride and which is blue in colour, changing to pink when waterhas been adsorbed. Humidified gel is silica gel that has been saturatedwith water from the vapour phase in order to avoid decrepitation uponimpregnation. If the cobalt chloride solution is added directly to thedried gel, the grain size is reduced.

Cobalt chloride has recently been classified as a Category 2 carcinogen(notification from the EEC, 15/12/98) with the consequence that the useof cobalt chloride indicator gel in industrial applications will requiremuch tighter control to ensure exposure limits are strictly controlled.If acceptable alternatives to the cobalt chloride indicator gel were notavailable to indicate when saturation had occurred in gas/air dryingapplications, for instance, this could have serious implications on theusers' downstream processes, e.g. corrosion through moisture damage.

U.S. Pat. No. 2,460,072 and U.S. Pat. No. 2,460,067 also disclosecopper(I) chloride and copper(II) bromide, respectively, but thesecompounds are not considered suitable candidates for a commercial silicagel-based humidity indicator because of potential toxicity andenvironmental considerations.

It has been demonstrated that the vanadium compound VOCl₃, whenimpregnated into silica gel gives a colour change from colourless toyellow to orange to red to brown as humidity increases—see the followingreferences:

-   Belotserkovskaya et al., “Indicator properties of vanadium-modified    silicas and zeolites” Zh. Prikl. Khim. (Leningrad), 63(8), 1674-9;-   Malygin, A. A. “Synthesis and study of physicochemical properties of    vanadium-ncontaining silica—a humidity indicator”, Sb. Nauch. Tr.    VNII Lyuminoforov I Osobo Chist. Veshchestv, 23, 24-8; and-   Malygin, A. A. et al, “Study of properties of vanadium-containing    silica gel”, Zh. Prikl. Khim. (Leningrad), 52(9), 2094-6.

However, VOCl₃ is corrosive, toxic and difficult to prepare and handle.

The present invention therefore addresses the problem of producing analternative, safe indicator gel to those which are cobalt-based orcontain a transition metal salt which is considered to be toxic.

According to one aspect of the present invention there is provided ahumidity indicator compound comprising a silica-based carrier containingan iron(II) and/or iron(III) salt or salts as the active indicator.

A second aspect of the present invention resides in the use, as ahumidity indicator, of a compound comprising a silica-based carriercontaining an iron(II) and/or iron(III) salt or salts as the activeindicator.

According to a third aspect of the present invention there is provided amethod of monitoring the humidity level within an atmosphere comprisingexposing a dried silica-based carrier containing an iron(II) and/oriron(III) salt or salts to said atmosphere and observing colour changestherein.

Typically, humidified silica gel is used as the carrier; however otherforms of silica may be used in the production of the silica-basedcarrier, e.g. silica hydrogel or dry silica gel. The silica-basedmaterial may have any of the physical forms normally available. Inparticular, the form can be irregular granules or approximatelyspherical beads (often called spherical or beaded silica gel).

The presence of the iron salt imparts a yellow or amber colour to thedry silica-based carrier. When the indicator is exposed to moisture, itthen adsorbs water and the colour is observed to fade until it becomesalmost colourless when the silica-based carrier is almost saturated withwater. This effect has been observed to be a general effect for thoseiron salts which have been examined.

Once the extent of exposure of the indicator to moisture has resulted ina change of colour from yellow/amber to almost colourless thesilica-based carrier may be processed, e.g. by heating, to restore itscolour, and re-used for humidity monitoring.

The effect referred to above, i.e. colour change from yellow or amber toalmost colourless, has been observed in all iron salts examined, e.g.simple iron salts such as ferric sulphate, ferric chloride or ferricnitrate and salts having at least two cations of which one is iron(II)or iron(III), examples of which are ammonium iron(III) sulphate,ammonium iron(II) sulphate and potassium iron(III) sulphate. The effecthas been found to be particularly pronounced for the double sulphates oralums.

While not wishing to be bound by theory, the effect is thought to berelated to hydrolysis and formation of coloured, polymeric Fe-hydroxyspecies. In dry silica, such species are thought to be polymerised, andalso bound to the silica, to a greater extent than in humidified gel.The higher the degree of polymerisation, and possibly also bonding tothe silica, the more intense the colour.

The effect would appear to be related to pH. Those iron salts whichexhibit higher pH values when dissolved in water give more intensecolours, and more pronounced colour changes, than those which exhibitlower (i.e. more acidic) pH values, possibly due to a higher degree ofpolymerisation of the Fe-hydroxy complexes. Thus ammonium iron(III)sulphate at 10% by weight in water has a pH of 1.7 and produces asilica-based carrier with a deep amber colour, whereas a 10% solution offerric chloride has a pH of 1.3 and results in a paler yellow shade. Thecolour of the simple salts can be enhanced by adjusting the pH to highervalues, comparable to the alums. This may be achieved by the addition ofsmall amounts of sodium hydroxide solution.

Normally an iron(III) salt is employed; however, ferrous counterparts ofthe ferric salts may also be used since the ferrous ion readily oxidisesto the ferric state.

Typically, the silica gel used has a BET surface area in the range of200 to 1500 m²/g. The pore volume of silica gel may be in the range of0.2 to 2.0 ml/g, as measured by nitrogen absorption. For example,Sorbsil desiccant gel (Sorbsil is a Trade Mark of Crosfield Limited)typically has a surface area of about 800 m²/g and a pore volume ofabout 0.4 ml/g. Surface area is determined using standard nitrogenadsorption methods of Brunauer, Emmett and Teller (BET).

The amount of iron present in the silica-based carrier is preferably atleast about 0.01 percent by weight of iron, determined as Fe, relativeto the dry weight of the carrier, typically up to about 2.0 percent andusually in the range of about 0.01 percent to about 1.0 percent byweight of the dry weight of the silica-based carrier. The dry weight ofa prepared humidity indicator, based on silica gel, according to theinvention can be determined by placing a weighed sample (approx. 20grams) in an oven at 145° C. for 16 hours and then weighing the driedmaterial.

According to another aspect of the present invention there is provided amethod of producing a humidity indicator comprising soaking silica-basedcarrier with a solution of an iron(II) and/or iron(III) salt to secureimpregnation of the carrier and drying the impregnated carrier.

Typically the indicator gel is prepared by contacting the silica-basedcarrier with a solution of iron salt containing 1 percent by weight orhigher (up to the saturation point) of the iron salt, e.g. by soakinghumidified white silica gel in the iron salt solution. Humidified gel ispreferred, but the use of dry gel is acceptable. When dry gel is used,the granules decrepitate, so that the product has a smaller particlesize than the original product, but, generally, the particle size isstill satisfactory for use as a drying agent. In the case of ammoniumiron(III) sulphate (herein referred to as iron(III) alum), the solutionmay range from 1 percent to approximately 50 percent by weight(saturation at 25° C.), or higher at higher temperatures. Preferably,the solution contains 10 to 40 percent by weight iron(III) alum at 25°C. The use of a high concentration of iron salt helps to reduce theprocessing time for preparing the indicating silica-based product. Thegel is typically soaked in the solution for a period of from 10 minutesto 10 days, preferably 1 to 30 hours, more preferably 2 to 24 hours. Theexcess solution is drained and the gel dried at 105 to 230° C. whereuponit develops its amber colour. An impregnated product dried in thismanner will usually have a weight loss after heating at 145° C. for 16hours of less than 10 percent by weight. Preferably, the weight loss at145° C. is less than 2 percent by weight.

The invention is illustrated by the following, non-limiting examples.

EXAMPLE 1

Sorbsil silica gel (commercially available from Crosfield Limited ofWarrington, England) was exposed to humidity or steam until the poresystem was totally saturated with water transported from the vapourphase. 50 g of this humidified gel was impregnated with a ferric salt bysoaking the gel in 200 ml of 20 percent by weight iron(III) alumsolution for 24 hours. The gel was drained and then dried at 145° C. for16 hours. 6 g samples of the impregnated, dried gel were placed in aseries of glass tubes and air at various levels of relative humidity(RH) passed through the gel for 7 hours at a flow rate of 4litres/minute. After exposure to moisture-containing air for this lengthof time, the colour of the gel samples was measured using a MinoltaCR200 Chromameter, calibrated to a standard white plate and using CIEIlluminant C and a 2° observer angle. The results, expressed accordingto the L*, a*, b* system, are given in Table 1 below.

TABLE 1 % RH % weight gain L* a* b* 0 0.0 39.96 10.96 32.38 20 8.4 44.408.95 35.47 40 14.9 48.13 4.39 23.53 50 18.2 50.00 1.67 15.90 80 25.659.94 −0.30 12.30

The increase in lightness (L*) and decrease in redness (a*) andyellowness (b*) is apparent from the above data and is readily observedvisually, allowing an obvious indication of when the gel has becomesaturated with moisture. Visually, the gel appears almost colourlessafter exposure to 50% RH air at 4 litres/min for 7 hours.

EXAMPLE 2

A further batch of silica gel was prepared according to the method ofExample 1 and the gel was soaked in the ferric alum solution for 4hours, rather than 24 hours. The product was similarly exposed to moistair and the results are given in Table 2 below.

TABLE 2 % RH Colour % weight gain L* a* b* 0 Deep amber 0.0 40.77 +13.2335.13 20 Pale amber 11.7 45.62 +6.94 36.90 40 Yellow 20.6 56.48 +0.5923.94 50 Pale Yellow 25.4 53.64 −0.28 17.90 80 Almost colourless 31.057.93 −1.74 14.28

This product shows an improvement over Example 1 especially in terms ofwater adsorption capacity.

EXAMPLE 3

Samples of ferric salts were placed in the oven at 145° C. for 16 hoursto observe the effect of dehydration and to see if any colour changesobserved matched those obtained with silica-based carriers impregnatedwith the ferric salts. Observations are given below in Table 3.

TABLE 3 Salt Colour prior to drying Colour after drying Ferric alum palelilac light buff Ferric sulphate light buff light buff Ferric chloridedeep yellow dark brown (decomposed to ferric oxide) Ferric nitrate paleviolet dark brown (decomposed to ferric oxide)

Where colour changes were observed, it was found that they did notcorrespond to those seen in the impregnated carrier. This indicates thatthe colour change observed in the carrier impregnated with iron(III)salts is not due to a simple hydration/rehydration effect as with cobaltand copper salts.

EXAMPLE 4

Silica gel was impregnated with various iron salts using a similarmethod to that described in Example 1. The details of the reactionconditions are given in Table 4 below. In these laboratory experimentsparticular care was taken to remove as much excess solution as possiblefrom the gel using tissue before the oven drying. During oven drying thetreated materials were spread in as thin a layer as possible. This wasfound to give a product with a more homogenous colour.

TABLE 4 Solution Ratio gel/ Soaking Sample Iron salt strength solutiontime A Potassium iron alum 10%  50 g/200 ml  24 hours B Ferric sulphate40%  50 g/200 ml  24 hours C Ferric chloride 10% 100 g/200 ml 2.5 hoursD Ferric nitrate 10% 100 g/200 ml 2.5 hours

The samples were exposed to moist air as described in Example 1 and theresults are given in Table 5 below.

TABLE 5 Sam- % ple RH % moisture Colour L* a* b* A 0 0 Amber 50.97 +7.54+35.06 20 10.5 Amber 49.55 +6.61 +34.73 40 20.9 Yellow 54.34 +2.00+25.10 50 23.6 Almost colourless 59.63 −0.23 +17.80 80 26.1 Almostcolourless 60.20 +0.29 +18.02 B 0 0 Yellow/amber 41.94 +4.15 +29.37 2011.0 Yellow 53.22 +2.55 +30.21 40 19.4 Yellow 53.75 +1.32 +27.22 50 21.5Pale yellow 57.67 +1.14 +26.74 80 23.8 Almost colourless 55.01 −0.32+22.54 C 0 0 Amber 39.02 +10.12 +32.63 20 11.2 Yellow/amber 48.05 +4.20+28.30 40 23.5 Pale yellow/amber 52.35 +2.88 +25.76 50 27.1 Paleyellow/amber 54.62 +1.79 +23.67 80 31.5 Pale yellow/amber 55.45 +1.46+23.29 D 0 0 Amber 44.49 +7.17 +31.10 20 10.8 Pale amber 45.79 +6.47+29.90 40 23.2 Pale amber 53.71 +4.14 +28.64 50 26.6 Pale amber 51.47+3.62 +26.16 80 29.6 Pale yellow 53.74 +2.68 +25.34

The chloride and nitrate show a less marked colour change than when alumis used. Nevertheless some lightening of colour is visible to the humaneye and the trend can still be detected using the L*a*b* system.

EXAMPLE 5

50 g of humidified gel, prepared as in Example 1, was soaked in 200 mlof a 20 percent by weight solution of ammonium iron(II) sulphate for 4hours and dried as in Example 1. The product was exposed to moist air asin Example 1 and the observed colour changes are shown in Table 6 below.

TABLE 6 % RH % water absorbed Colour L* a* b* 0 0 Amber 44.09 +15.81+43.11 20 11.4 Amber 44.97 +14.44 +40.84 40 23.4 Pale amber 52.82 +8.43+36.41 50 26.6 Pale yellow/ 52.05 +6.08 +32.99 amber 80 30.0 Pale yellow57.91 +2.62 +27.56

Ammonium iron(II) sulphate has the normal green colour of ferrous salts.However, silica gel integrated with it and dried has the amber colourassociated with iron(III) salts.

EXAMPLE 6

Samples of commercially available beaded silica gel from three supplierswere humidified and then impregnated with 20% iron(III) alum solutionfor 7 hours, dried at 145° C. overnight and the colour recorded. Sampleswere then placed in a desiccator at 100% relative humidity for a weekand the colour recorded.

The desiccant silica gel beads used in this experiment, and theirsuppliers, were:

Bead type/size Supplier “TS6”, 2-5 mm QingDao HaiYang Chemical Group Co.Ltd., 7 Mian Yang Road, QingDao, China. 2-5 mm Silgel Packaging Ltd., 2Horton Court, Hortonwood 50, Telford, Shropshire, UK. Ca. 1-3 mmEngelhard Corp., 600 E. McDowell Road, Jackson, MS 39204, U.S.A.

Colour changes for the desiccant beaded silica gel impregnated withiron(III) alum are given in Table 7 below.

TABLE 7 Supplier Colour L* a* b* Haiyang before Amber 46.05 +10.10+37.91 exposure after exposure Almost colourless 53.10 −1.58 +18.05Silgel before Amber 48.69 +13.64 +43.57 exposure after exposure Almostcolourless 61.07 −2.16 +16.04 Engelhard before Amber 50.27 +16.18 +51.36exposure after exposure Almost colourless 58.03 −1.35 +15.50

In each case the beaded silica gel shows a pronounced colour change fromamber when dry to almost colourless when humidified. This is the samebehaviour as is observed when an irregular grannular silica gel is used.

EXAMPLE 7

Dry silica gel, when placed in water (or an aqueous solution), is knownto decrepitate. However, decrepitation is not necessarily a problem inthe preparation of acceptable indicating silica gel. To demonstratethis, 50 g dry silica gel having a size range of approximately 2.5 to6.0 mm was soaked in 200 ml of 20 percent by weight iron(III) alumsolution for 4 hours and then dried at 145° C. overnight. The colour ofthe resulting gel was measured before and after being left in adesiccator at 100% relative humidity for 3 weeks. The colour changes areshown in Table 8 below. A sieve analysis was carried out before andafter the impregnation step to demonstrate the effect of decrepitationon particle size distribution. The results are shown in Table 9 below.

TABLE 8 Colour L* a* b* Before exposure Amber 58.32 +9.97 +53.29 Afterexposure Pale yellow 66.67 −2.84 +20.06

TABLE 9 Particle Weight % before Weight % after size (mm) impregnationimpregnation >5.6 4.74 0.00 3.55-5.6  63.31 0.71  1.6-3.55 31.91 36.171.0-1.6 0.04 36.35 0.5-1.0 0.01 22.40 <0.5 0.00 4.37

There had been some breakdown in particle size as a result of thedecrepitation but this does not interfere with the humidity indication.The gel in this example still showed the expected amber to nearlycolourless colour change and the particle size distribution was stillacceptable for normal desiccant use.

EXAMPLE 8

100 kg of humidified 2.5-6.0 mm silica gel prepared as in Example 1 wassoaked in 180 litres of 20 percent by weight iron(III) alum solution for4 hours. A pump was used to keep the solution circulating at a rate ofbetween 25 and 50 litres per minute. The gel was then withdrawn, allowedto drain and then dried at 150° C. overnight in 2 cm deep trays in anoven. Colour and adsorption capacity were measured for the freshmaterial as in Example 1. Analysis showed it to contain 0.34% Fe.

200 g of orange indicator gel, made as described above, were placed in abowl in a desiccator containing water. The relative humidity in thisdesiccator was nearly 100% at 25° C. After about two weeks exposure tothis high humidity the gel had decolourised. It was then oven dried at145° C. overnight and the process repeated. This exposure andregeneration was carried out ten times. After the ten cycles of humidityand drying the colour and adsorption capacity of the gel were measuredagain as in Example 1 and compared to the original material. The resultsare shown below in Tables 10 and 11.

TABLE 10 Effect on colour. Sample L* a* b* Fresh before exposure 42.80+11.28 +37.71 after exposure 61.92 −1.02 +15.18 After 10 cycles beforeexposure 41.74 +11.48 +37.86 after exposure 56.72 −1.64 +12.58

TABLE 11 Effect on adsorption capacity. Sample % RH % water adsorbedFresh 20 11.6 40 22.9 50 28.0 80 31.2 After 10 cycles 20 10.1 40 22.5 5027.6 80 30.8

Visually, the fresh and regenerated gels were indistinguishable. Therehad been no deterioration in colour shade, intensity or distribution andthe colour change effect was also uneffected. In addition, theadsorption capacities show no adverse change after ten regenerationcycles.

Similar recycle results can be obtained with other iron salts but it hasbeen found that the drying temperature may need to be kept below about100° C. when certain salts (e.g. FeCl₃) are used, in order to avoid thedevelopment of an uneven colouration.

1. A humidity indicator comprising a silica gel carrier containing aniron(II) and/or iron(III) salt or salts as an active indicator, the ironsalt being a salt containing more than one cation of which one isiron(II) or iron(III) wherein the silica gel carrier changes color whenexposed to humidity.
 2. An indicator as claimed in claim 1 in which theiron salt is ammonium iron(III) sulphate, ammonium iron(II) sulphate orpotassium iron(III) sulphate.
 3. An indicator as claimed in claim 1 inwhich the iron(II) or iron(III) salt is present in an amount in therange 0.01 percent to 2.0 percent by weight expressed as Fe based uponweight of dry carrier.
 4. A humidity indicator as claimed in claim 1 inwhich the silica gel is humidified silica gel.
 5. A humidity indicatoras claimed in claim 1 in which the silica gel is beaded or granularsilica gel.
 6. A method of monitoring the humidity level within anatmosphere comprising exposing a dried silica gel carrier containing aniron(II) and/or iron(III) salt or salts to said atmosphere and observingsaid silica gel carrier for color changes therein, the iron salt being asalt containing more than one cation of which one is iron(II) oriron(III).
 7. A method as claimed in claim 6 in which the iron salt isammonium iron(III) sulphate, ammonium iron(II) sulphate or potassiumiron(III) sulphate.
 8. A method of producing a humidity indicatorcomprising soaking silica gel carrier with a solution of an iron(II)and/or iron(III) salt or salts to impregnate the carrier and drying theimpregnated carrier, the iron salt being a salt containing more than onecation of which one is iron(II) or iron(III) wherein the dried carrierchanges color when exposed to humidity.
 9. A humidity indicator in theform of an iron impregnated silica gel carrier produced by the method asclaimed in claim
 8. 10. A humidity indicator composition comprising asilica gel carrier containing ferric sulphate wherein said ferricsulphate is present in an amount in the range 0.01 percent to 2.0percent by weight expressed as Fe based upon weight of dry carrier andwherein said dried carrier changes color when exposed to humidity.
 11. Amethod of monitoring the humidity level within an atmosphere comprisingexposing a dried silica gel carrier containing ferric sulphate to saidatmosphere and observing color changes therein.
 12. A method ofproducing a colored humidity indicator comprising soaking a silica gelcarrier with a solution of iron sulphate to secure impregnation of thecarrier and drying the impregnated carrier wherein said iron sulphate ispresent in an amount in the range of 0.01 percent to 2.0 percent byweight expressed as Fe based upon weight of dry carrier, and whereinsaid dried carrier changes color when exposed to humidity.
 13. A methodas claimed in claim 12 in which the color of the indicator is enhancedby pH adjustment to higher values.
 14. A method as claimed in claim 13in which pH adjustment is effected by sodium hydroxide addition.
 15. Amethod as claimed in claim 12 including incorporating into said silicagel carrier an alkali to adjust pH to higher values.
 16. A coloredhumidity indicator compound comprising a silica based carrier containinga simple iron salt as an active indicator and in which the color of saidhumidity indicator is enhanced by the addition of a base.
 17. A compoundas claimed in claim 16 in which the iron salt comprises ferric sulphate.18. A compound as claimed in claim 16 in which the carrier comprisessilica gel.
 19. A compound as claimed in claim 18 in which the silicagel is humidified silica gel.
 20. A compound as claimed in claim 18 inwhich the silica gel is beaded or granular silica gel.
 21. A method ofmonitoring the humidity level within an atmosphere comprising providinga dried silica based carrier which contains a simple iron salt as acolored indicator and in which the color of the indicator is enhanced bythe addition of a base, exposing said carrier to said atmosphere andobserving color changes therein.
 22. A method as claimed in claim 21 inwhich the base is sodium hydroxide.
 23. A method as claimed in claim 21in which the iron salt comprises ferric sulphate.